Ophthalmic lens and process of making the same



Jan.. 16, 1940. E. D. TILLYER v DPHTHALMIC LENS PROCESS OF MAKING THESAME Original Filed April 14, 1934 il. ens 5y.: tem-L ,lgllll v. lLiVENTORv J1 Il et. .4% RNEY l 7 ZI ma, 19

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20 OC mwhm Patented Jan. 16, 1940 UNITED STATES oPn'rHALMIc LENS ANDPaocsss oF MAKING Tm.: SAME Edgar D. Tillyer, Southbridge, Mass.,assis-nor to American Optical Company, Southbridge, Mass., a voluntaryassociation of Massachusetts Original application April 14, 193.4,Serial No.

720,594. Divided and this application March 29, 1937, Serial No. 133,599

4 Claims. (Cl. 88-54) This invention relates to improvements inophthalmic lenses and to an improved process for making the. same, andrelates particularly to ophthalmiclenses used for the equalization ofthe mental impressions of size in the two eyes said impressions beingalso referred to in the art as ocular images.

This application is a division of my Tillyer Patent No. 2,077,134,issued April 13, 1937.

One of the principal objects of the invention ris to provide means ofseparating in a lens or lens system the size and the focal powerfactors, and in a factor form, so one surface is left for the impressionof the prescriptive focal power required, and the remaining parts. togive the true size effect independently of the said prescriptivesurface.

Another object of the invention is to provide means whereby size andpower lenses or lens systems may be supplied the dispenser in .such formthat the lens or lens system may be nished by him to requiredprescriptive power by simply impressing on one surface the saidprescriptive power curve, thereby making it possible to dispense theselenses or systems in the same way ordinary ophthalmic lenses aredispensed in the art, instead of requiring the whole lens or lens systemto be made to required prescription by a lens factory which would delaythe time in providing the desired lenses to the patient and wouldmaterially increase their cost to the patient.

Another object' of the invention ls to provide a method of computationfor lenses and lens systems of this character which takes into accountall of the factors involved in lenses or lens systems of this nature andseparates out the power effect, the true size effect, and the variationsfor distance from the eye of the lens or lens system and the distance tothe object and eliminating latery those of vanishing importance, wherebyany prescriptive lens of this nature may be expressed by formula andreadily designed therefrom, differentiating from prior methods ofcomputation where individual prescriptions were each figuredindependently for their individual condition of use, whereby I am ableto codify and systematize the entire range required for use for ordinaryand usual prescriptions, instead of having to compute laboriously andexpensively each individual lens or lens system requiring a laboratorycomputation and factory production of each required lens or lens system.

Another object of the invention is to provide lens blanks for lenses andlens systems of this character in semi-nuished form codied for truemagnification or size factor whereby they may be supplied vin series ofvarious magnications which may be completed for required prescriptivepower by the dispenser simply by impressing the prescriptive power curveon a surface left therefor.

Other objects and advantage of the invention will `become apparent fromthe followingi description taken in connection with the accompanyingdrawing. It is apparent that many changes may be made in the details ofconstruction and arrangement of parts and in the steps of the processwithout departing from the spirit of the invention as expressed in theaccompanying claims, the preferred forms, steps, and arrangements beingshown yand described by way of illustration only.

Referring to the drawing:

Fig. I is a diagrammatic illustration of a size lens system placedbefore the eye and viewing an object for the purpose of deriving thegeneralized formula for lenses and lens systems of the invention;

Fig. II is a cross section of a lens blank ofthe invention showing asurface left free for impression of lthe prescriptive power curve;

Fig. III is av view similar to Fig. IIl of the same lens blank with adefinite prescription power curve impressed on the free surface;

Fig. IV is a view similar to Fig. II, with a prescriptive curve on thefree surface different from that of Fig. 1".I but having the same valueof true magnification S1, but a different effective power De;

Fig. V is a cross section of a tw'o element lens system showing a freesurface for the prescriptive power curve;

Fig. VI is a top diagrammatic sectional view of a pair of spectacles oreyeglasses having lensA systems of the invention;.

Fig. VII is a cross section of two lenses cemented together for thepurpose of aligning the axes on the two outer surfaces;

Fig. VIII is a cross section of two lenses with an air space between;and

Fig. IX is a cross section of a modified form of the invention showing aseparator or a filler piece between the lens elements to provide forincreasing or decreasing the space between said elements withoutappreciably increasing the weight of the finished lens.

In the prior art. single vision and multifocal ophthalmic lenses havebeen dispensed by the factory finishing one side of the lens blank.These blanks were sold to the dispensers who resii the unfinished sideof the blank a surface to complete the lens to prescriptionrequirements. This simplified and cheapened the dispensing of suchlenses and saved time for the patient, making one day service to thepatient possible. It is clear that it is very impracticable for afactory to have to make individual prescriptions for patients. It wouldbe a lens by lens job, would be very expensive for each case, and wouldcause almost indefinite delay in delivery. 'I'he system described aboveis the universal system of the art in dispensing such lenses and hasthrough practice been reduced to a very efiicient and practical one.

Up to a very short time ago, ophthalmic lenses provided only for focalpower, defect in shape of eye, astigmatism and muscular defects,prismatic displacement, utilizing the spherical curve for power, acylinder or torlc curve for astigmatism, and prism for muscular defect.Recently, however, it has been discovered and revealed to the art thatthere may also be defects in size impressions of the two eyes, one eyemay see larger than the other, or a single eye may have a different sizeimpression in different meridians. This defect has been compensated forby adding a magnification factor in the lenses or lens systems, one thatwill change the size impression or relative size of the ocular imageswithout aecting the focal power of the lens systems. This factor isintroduced by means of the property of lenses to change magnificationwithout change of focal power by means of a change of shape, thickness,distance from the eye, and distance of the object from the eye; it is afactor of the shape or form of the lens and not merely its focal power.The prescribing4 of such lenses is in its infancy. Up to the presenttime it has been confined to computing the lens in the laboratory andfinishing the lens completely at the factory for each individualprescription, a very costly, laborious, and lengthy proceeding, and oneimpracticable to the organized methods of making and dispensingophthalmic lenses. It is a principal obiect of my invention to avoidthese expenses, delays, and laborious proceedings involved in making anddispensing lenses of this nature, by providing a simplified andgeneralized method of computing such lenses by a general formula which Ihave derived so the same may be codified and systematized to ll theusual and general prescriptions in the usual methods of dispensing nowin vogue in the art, simplifying and cheapening the computations as wellas the methods of production and dispensing by providing the dispenserwith lens blanks as in the present system which may beV converted intofinished lenses of required magnification and focal power by simplyimpressing a power surface on one surface of the blank left free forthat purpose by the manufacturer, and to provide such blanks in seriesof varied magnifications which may be utilized by the dispenser to meetthe prescriptive requirements of individual prescriptions presented tohim embodying the correction of size as well as focal power andastigmatism where the combination of the two is required. My inventionembodies both new computations and methods of computation as well as anew method of producing and supplying lenses and blanks of thischaracter.

The majority of size lenses fall within the range from no truemagnification to four per cent true magnification.

I assume for the sake of deriving a formula for the image size that theeye is static. At rst,

I assume an object at any distance and determine rigorous formulae, thenassume distant object. then show how distant formulae can beapproximately corrected to near object when rigorous near formulae aretoo complicated for practical use.

Ii' the eye, in looking at a distant object, has any kind of a lenssystem in front -of it, then the bundle of light rays, forming eitherimages or blurred images (if the focus is not sharp), passes through axed opening which is the image of the pupil of the eye as formed by thecornea; in other words, assume that there is a fixed opening properlyplaced like a stop. We can then derive the following expression for themagnification of this lens system:

l l M""1-U1JXc Where M1=the complete magnification for a distant object.

U :the distance from fixed opening (entrance window) to ocular surfaceof the lens system,

De=the effective power of the lens system.

C =A function of all the surfaces, refractive indices of the glass,thicknesses, and separations, except the power of the ocular surface.

An analysis of this formula shows that the complete mafinification for astatic eye and distant object is the product of two independent terms,namely, the first term gives the magnification eect produced by thefocal power, and the second gives the magnification effect of theshapes, separations, thicknesses, etc., i. e., the magnification, exceptthat the last ocular surface is left free so a surface may be placed onit to give the desired prescriptive power.

I indicate the distant magnification due to power P1, and themagnification due to shape or form S1, then:

191:@ and S1= and 1 1 1 UD, XE

The examination of the eyes for their errors, obviously, must determinetheir refractive correction which is De and which, because ofunavoidable thickness of the test lenses does contain some C or S1 butwhich can be allowed for, and likewise must contain the complete powermagniflcation P1.

Next let us consider U so we can determine P1. It is measured from apoint roughly four millimeters on the retinal side of the cornea to theocular surface of the test lens, or from the center of rotation of theeye, if the eye rotates but we need not know its accurate value, infact, if we put the ocular surface of the prescription lens at the sameplace as the ocular surface of the test lens we do not need its value atall, since P1 for the prescriptive lens will be the same as P1 for thetest lens system. Roughly, U is twenty millimeters, since everything inlens theory is expressed in meters, U=0.02O meter.

If the test lens is placed at a different position than the prescriptionlens there is a change in P1 due to change in U, this change in U mustbe known accurately but not the actual value of U, likewise De must bechanged or corrected, as is well known, to compensate this changebecause Mizpixsi:

the effectivity of focal power of the lens changes as its distance fromthe eye is changed.

This leaves S1 or the true magnification uncontaminated with varyingdegrees of corrected eye focus. The commercial importance of fixed; achange in the position of the prescription in front of the eye isachange in U. For the effect of a ychange in U, we can make anapproximation 'to our exact equation:

P1=1|UD approximately, or in percentage and millimeters:

l(P1-1) in percentage =1A.%, times the change in U in millimeters for avalue of De of 2.50 diopters (an average prescription).

Since we must keep the product PXS constant, we must change S if Wechange P. -If De is more,

- the change is more and vice versa.

The determination of the positionv ofthe test lens and the prescriptionlens must be accurate.. The most accurate measurement that can bephysicallymade is from the front surface of the .y

prescription lens to the front surface of the cornea, but U should bemeasured from the ocular surface, i. e., the `ocular surface of the testlens should be positioned the same with respectv to the cornea or theocular surface of the prescription llens as is common ophthalmicpractice.

It will be seen from the formula v that in the part of the expression Ihave collected all of the elements involving the focal power of the lenssystem, expressed as D. the effective power of the lens as it isordinarily measured combined with a distance U which indicates theposition the lens is placed before the eye, while in the portion I havecollected all the elements which are independent of the position of thelens and out of which I have kept or excluded one surface, i.` e., theocular surface, which makes it possible for me to change the said ocularsurface at will to get a desired value of the power De without changingthe true size magnification represented by and which is called hereinS1.

It is because of this separation of the equation into the two groupsthat I am able to provide lenses on which different powers of ocularsurfaces may be imposed without affecting the true magnification of thelens, thereby making it possible to dispense the lenses in the usual waythat ophthalmic lenses are dispensed by the dispensers in that art,which has hitherto been considered impossible.

The free or excluded surface which I have is ordinarily the ocularsurface of the lens system: i. e., the surface nearest the eye.

My method of analysis is based on the principles of Gauss as laid downby Pendlebury in 1884 with my necessary extensions to that theory. l

The following considerations and symbols are used in this analysis:

'I'he direction of incident light, left to right is positive. All raysmeasured in this direction are positive.

A radius of curvature, convex to the incident light is positive.

The order of indices of the refractive medium are indicated. no, p1, nzetc. (n beingv the Greek letter mu).

Surface powers,

. I'i rz (p being the Greek letter rho).

Thickness r (the Greek letter tau) positive; reduced thickness arenegative, but when s is used for a reduced thickness it is positive;likewise, when D (diopter) is used in place of p (rho) it has thevconventional value as ordinarily used in ophthalmic practice. Thereason for the adoption of the above arbitrary negative values of thereduced thickness tf is to keep all they signs in a. convenient form.

Note that De also the D which refers to surface powers' is not the A D-piP Then if we call the part of the vmagnification for a distant objectdue to the power D and that due to shape S1 then 4. ...1 P1-1 UD,s1-C,P1s Ml De is what is commonly called the effective power or vertexrefraction of the lens and is actually the reciprocal of the back focallength of the lens expressed in-meters.

The form ofv A, B, C, D is obtained as follows is an expressionindicating the partial derivative of A with respect to p1; etc.

by means of which the equation for n surfaces can be obtained.

For wo surfaces AJ=p1+p2+p1pz C=|+p1t or in terms of D and s ophthalmicnotation For four surfacestwo separated lenses n=4 By means of precedingformulae any number of surfaces may be obtained.

Four surfaces- Important terms The important terms for a distant objectare collected below (n is an air space between lenses) 1 UB-AUd-l-Cd-Dapproximately for four surfaces; since Therefore the quantities exceptthe p4 already compensated for in the design. This leaves only which isalready eliminated when U is the same with the test lenses as with theprescription lenses and (pi-i-pz+pa+p4) is the approximate value of theactual power prescription, and the terms Up4(t1+tz+f3) +Up42r which isequal to which cannot be completely compensated for in the semifinishedblanks since p4 will vary with the power of its prescription. Howeverlet us limit the range of powers over which a given semnished series ofblanks is to be used, then we will know the approximate value of thisterm.

For an extreme range we can take p4 to vary from 5D to 15D which is 5Deach side of the mean, then we can take a=1.5, the total thickness 21-as 0.010m and U=0.02m then the error in A-is 0.02 X 5 X 0.010X 0.3 0.4

or if this is reduced to per cent in magnication we have 0.08% which iscloser than required and can be further reduced if desired.

It is thus seen that seminished blanks can be made practically so thatthe prescription curve can be placed on one surface for near as well asfor distance.

For the discussion of Pendlebury referred to above see Lenses andSystems of Lenses treated after the manner of Gauss, by CharlesPendlebury, M.A., F.R.A.S., Published, Cambridge, England, 1884."

Referring to Fig. I the following is an explanation of thelsymbols used:

Object lis imaged into lm.

Distance object to lens system=d=-u.

Distance image to last surface of lens=v.

Angular size of object from stop point wo.

Angular size of image fromstop point wn.

Magnification (angular) =tan wo Distance stop from lens system U. Totalthickness of lens system Er. Linear magnication=mbut from Pendlebury A,B, C and D are expressions from Pendlebury and are given later.

But if the object is at a distance, d is large in comparison witheverything else. Call the magnication for a distant object M' instead ofM, vwe have and C does not contain the last surface.

The lens shown in Fig. II comprises a lens element having the surface 1and a thickness greater than 1, say 1 -I- where :z: is sufficient extraglass so that the lens may be finished to the thickness f. The surface 1is finished optical surface. 'I'he surface 2 may be left uniinished fora purpose to be described later.

To start with, say we desire a lens having a certain S1, or true sizemagniiication. Then we have 1 Sll-SDl where D1 is the surface power of 1and sis the thickness 1 divided by the refractive index of the glass. Itwill be seen from this formula that either small or large values of scan be used provided we use with the small values, large values of D1and vice versa, so we choose a good average value of both s and D1 for aAcommercial lens, which values are so chosen as to satisfy the aboveequation.

Then we compute the eiective power or vertex refraction of the lens,assuming the surface 2 to be fiat or plano.

Then if we wish a lens with no focal power, we put on surface 2 thefocal power computed with opposite sign, as for example, if say, a fiatsurface 2 gives an effective power of plus 6 diopters, we would for azero power lens vgrind a minus 6 diopter surface curve on the face 2. Ifwe wished a power of plus 1 diopter, we would grind on the face 2 aminus 5 diopter curve and so on. Whatever the surface ground on the face2, the thick- I ness r must be preserved for the finished lens in orderto maintain the S1 value of the lens.

When the eye has been tested, the magnification due to the power of thelens has been placed in front cf the eye in the test lenses, so we donot need to include P1 of the formula, unless we wish to change thedistance the prescription lens is to be placed before the eye when it isto be other than that of the trial lens. When we do make this change ofdistance this obviously changes U in the formulal for Pl and must beallowed for.

In the test lenses, if they were very thin, they would involve no shapemagnification, but actually they are not very thin, so there ls some Slfor the test lens. This must be added on to the pure size measurementswhich have been made in order to get the'complete size difference S1. Ifthere are power ,test lenses in front .of each eye, then there is onlythe ratio of the S1 of one test lens to the Sl of the other to beallowed for.

In Fig. III there is shown the same lens as Fig. I, except that a powercurve has been placed on the face 2 to show a lens of zero power.

In Fig. IV there is shown a lens the same as Fig.

y I, but a different curve has beenplaced on the face 2 to give adifferent focal power to the lens, but al1 the lenses of Figs. III andIV have the same true size magnification S1.

`In Fig. V there is shown a lens system of two separate lens elements 3and 4, having surfaces 5, 6, 1, and 8, and thicknesses n, n, and fs.where '-rz is an air space. The surface 8 has been left unfinished, andthe actual value of n is -ra-I-z, as explained before, the :c to beground away when ysurface 8 is finished to required prescription to thelenses in Fig. II;

We have no De in this lens because it is a blank with surface I finishedaccording to the formlua and the thickness o f the mtimate lensdetermined by the same formula.

In Fig. III we have a lens with De equal to zero and the lens isfinished to the thickness r as described for Fig. II with a curve on theocular side 2 such that there is zero De power.

In Fig. IV we have the same lens finished to give a De of plus onediopter focal power.

To determine the curve I and the thickness of all these lenses, i. e.,Figs. II, III, and IV, we assumed that we required a magnification 1.8per cent, which makes S1=1.018-which is 1.8 per cent greater than unity.Then we have for the true size magnification (S1) equals one divided by1-s1 Di of the formula. Thus, if we take D1, the power of surface Iequal to plus six diopters s is 0.003, but s is the so calledireducedthickness, therefore, it must be multiplied by the index of refractionof the glass to get the actual glass thickness r of the finished lens,which is 0.003 times 1.52 equal to 0.0046 meters or as is commonlyexpressed 4.6 millimeters. Thus we have a front surface I of 6 dioptersand a thickness r of 4.6 millimeters. We have not carried that we wouldin actual lens design.

ill

If for other reasons we wish to make the surface I steeper or lesssteep, we can change f to correspond and get the same magnification, solong as we use the formula 1 smi-1Dl In Fig. III we have D=zero, so thatwe make surface 2 slightly stronger than surface I to make the effectivepower De=zero by the regular However, it is not necessary to computeP1since this part of the magnification due to De is already in the testlenses. Also, because of the finite dimensions of the test lenses thereis a small shape magnification due to their thickness and shape inaddition to the size correction from the size lenses, but in uniting theprescription the small shape correction of the test lenses is combinedwith the size correction found from the size lenses.

In the lens system of Fig. V we use the formula M1:P1 S1 where S1 iswhich has previously been given.

The prescription gives S1 and the power De. In this case we have anumber of surfaces D1, D2, Da, or as we have written them p1, p2, p3,also the reduced thickness is for the first lens, the air space which isthe second separation in the formula C7, and finally the third reducedthickness. These are many more quantities than are necessary for simplythe determination of the magnification, so we can impose otherconditions as required, and get the same true magnification, S1, fromthe formula of Cv. After the surfaces and thickness have been determinedto give the magnification S1, we can put the ocular surface on this lensto give the required value of De by the usual formula for effectivepower.

The lens system of Fig. V has the following characteristics.

Index of glass 1.5.

Radius of surface 5:50 millimeters, giving a surface power of plusdiopters.

Radius of surface 6:60 millimeters, or surface power of minus 8.33diopters.

Radius of surface 7:70 millimeters, or surface power of plus 7.14diopters.

Thickness f1=2 millimeters. giving the reduced t1 of minus 0.0013 meter.

Thickness 12:0.6 millimeter, giving reduced tz since this is an airspace=minus 0.0006 meter.

Thickness 73:3.5 millimeters, or a reduced thickness t3 minus 0.0023meter.

This surface 8 is to be determined by the power De desired in theprescription.

The lens was figured as follows from formula C7:

Therefore C7=1-0.0343=0.9657

and 81:1.0355

or the value of the true size magnification for this lens is 3.5%. Therequired power De, can be computed for any value of p4 or theI value ofp4 can be determined for any value of De required by well knowncomputations.

In Fig. VI there is shown a pair of lenses 9 and I0 mounted in a framebefore the eyes. Let us assume that the eye in front of which is mountedthe lens 9 requires a given amount of true magnification Sl over that ofthe other eye. There will be some unavoidable magnification in the lensI0, so we must make S1 of the lens 9 larger than S1l by this amount, asfor example, suppose the eye which the lens 9 is in front of requires atrue magnification of 1.02 and the lens I0 has a true shapemagnification of 1.01, then we must make the lens 9 to have a sizemagnification S1 equal vto the product of 1.01 multiplied by 1.02, whichgives about 1.03 for the shape magnification required; in other words,the ratios of the magnifications of the two lenses must be the requiredamount to give the right size correction to the eyes.

In Fig. VII there is shown a two element lens system composing theelements Il and I2 fitted together on their contacting faces I3 andsecured together by cement or otherwise to make a unitary lensstructure.

In Fig. VIII there is shown a two element lens system, comprising theelements Il and I5 with an air space I6 between them. The two elementsare fitted and secured together adjacent their marginal edges to form aunitary lens structure.

The structures of Figs. VII and VIII are particularly important wherethe true size magniflcation is different in one meridian than in theother, and in consequence requires a toric surface on a face of eachpart because the toric axes may be easily aligned after they arefinished by rotating one element on the other, it being a very difficultand expensive operation to align toric axes in one piece structures withsufficient accuracy.

In Fig. IX there is shown a modified form of the invention wherein thelens elements II and I8 are held in spaced relation by a spacer memberor ller piece IB of glass or other suitable means which is varied inthickness to increase or decrease the space between the lens elementsand thereby increase or decrease the magnification Without appreciablyincreasing the weight of the gramos vfinished lens. The edges of thelens elements I1 plied as single units for various values of S1 eitherspherical or toric, in the latter case there are two values of Sl foreach blank. A desired prescription may be filled by the dispenser bypicking out a blank with the desired S1 value and placing on the freeface the required prescription curve to give the desired focal power. v

The blanks may be also supplied in series of different magnications.graded to meet usual practical requirements.

The surfaces may be spherical, cylindrical, toric prismatic, aspheric orany of the surfaces of prior art lenses and ground and finished in theusual prior art way b'y prior art methods and for the general purposesof prior art corrections.

The lenses may be given any desired outline shape and will adaptthemselves to practically all of the usual prior art outline shapes.They may also be mounted in rimmed or rimless frames and mountings ofprior art construction in the usual prior art ways.

The denite reduction from a distant object to a near object is shown bythe formula set forth and can be applied where necessary but forpractically all the ordinary cases the reduction is so small as to beneglectable since it is less than the tolerance of the eyes.

No specific mention has been made of bifocal lenses but they falldirectly under the formulas given herein, except that there are threesurfaces often instead of four surfaces. The three surface formula isderived from the basic differential equations or may be derived from thefour surface equations by putting the first thickness equal to zero andthe first power equal to zero, and in case of a. fused bifocal,substituting the correct values of the indices of refraction that areactually used in the lenses.

The expressions, true magnification, or shape magnification, etc. areused for the magnification due to the shape and thicknesses, etc., asdistinguishedr from the power magnication that would be produced by aninfinitely thin lens having the same focal power as the lenscornbination actually used and placed at the same distance from thecornea as the ocular surface of the lens combination.

From the foregoing it will be seen that I have provided a newcommutation of lens systems of this character and have provided newlenses to give the desired corrective results by which the computation,manufacture, and dispensation of lenses of this character are materiallysimplified and cheapened, and by which service to the public ismaterially facilitated.

Having described my. invention, I claim:

1. The method of forming a blank, for an ophthalmic lens, of lens mediumof an index of refraction such as is generally used for ophthalmiclenses and which has a separately controlled shape magnificationcomponent S1 which distinguishes from the inherent power'mag'nificationdue to the focal power present in the finished lens, with the amount ofsaid shape magnification component dependent upon the front surfacepower factor D1 and the center thickness factor f of the finished lens.comprising choosing a value for one of said factors which is consistentwith the weight, appearance, and optical characteristics desired in thefinished lens, calculating the other 'of said factors by means of the.formula according to the index of refraction a of the lens medium usedfor said blank, the desired magnification component S1 and the saidchosen value for the first of said factors, and forming a front opticalsurface, to a curvature simulating the front curvature consideration D1of the calculation, on a piece of lens medium of said index ofrefraction having a center thickness which is greater than the saiddetermined thickness factor r of said calculation by an amount which issuch that after the said surface has been formed the center thickness ofsaid piece of lens medium will still kbe sufliciently greater than saiddetermined thickness factor 1- to allow the forming of a rear ocularsurface on said piece of lens medium, without reducing the centerthickness of said piece of lens medium below the said determinedthickness factor lr, which, in combination with said front opticalsurface, will produce the desired focal power ADe in the finished lens,said blank being adapted to be reduced to said determined centerthickness factor -r during the forming of said rear ocular surface so asto introduce the focal power De desired with substantially no change inthe amount of said separately controlled magnification component S1.

2. The method of making an ophthalmic lens. of lens medium of an indexlof refraction such as is generally used for ophthalmic lenses,'having ai si:

according to the index of refraction a of the lens medium used for saidlens, the desired magnification component S1 and the said chosen valuefor the first of said factors, forming a front opt-ical surface, to acurvature simulating the front curvature consideration D1 of thecalculation, on a piece of lens medium of said index of refractionhaving a center thickness which is greater than the said determinedthickness factor r of said calculation by an amount which is such thatafter the said surface has been formed the center thickness of saidpiece of lens medium will still be sufficiently Agreater than saiddetermined thickness factor 1 to allow the forming of a rear ocularsurface on said piece of lens medium, without reducing the centerthickness of said piece of lens medium below the said determinedthickness factor f, which, in combination with the front opticalsurface, will produce when combined with the front opticalA surface itwill introduce the focal power De desired in the finished lens andsimultaneously reducing the lens medium to the determined centerthickness factor r so as to introduce substantially no change in theamount of said separately controlled magnification component S1.

3. The method of forming a blank, for an ophthalmic lens system, of lensmedium of an index of refraction such as is generally used forophthalmic lenses, and which has a separately controlled magnificationcomponent S1 which distinguishes from the inherent magnification due tofocal power present in the finished lens system, with the amount of saidmagnification component dependent upon a factor D1 which is thecombination of all of the surface powers of the lens system except thatof the ocular surface, and a factor 2r which is the combination of thecenter thicknesses and the center separations of the finished lenssystem, comprising choosing values, to make up one of said factors,which are consistent with the weight, appearance, and opticalcharacteristics desired in the finished lens system, calculating thevalues to make up the other of said factors by means of the formula Cvbeing a function of all the surfaces, refractive indices of the glass,thicknesses and separations, except the power of the ocular surface,according to the indices of refraction p. of the pieces of lens mediumused for said system, the desired magnification component S1, and thesaid chosen values which make up the first of said factors, formingoptical surfaces on all of the surfaces of the lens system except theocular surface to curvatures simulating the surface power considerationsp of the calculation, on pieces of lens medium of said indices ofrefraction which have center thicknesses and separations r whichtogether make up a total center thickness which is greater than the saiddetermined center thickness and separation factor E1- of saidcalculation by an amount which is such that after the said opticalsurfaces have been formed on all of the surfaces of the lens systemexcept the ocular surface thereof, the total of the center thicknessesand separations of said pieces of lens medium will still be sufcientlygreater than said determined center thickness and separation factor 21to allow the forming of a rear ocular surface on one of said pieces oflens medium, Without reducing the total of the center thicknesses andseparations of said pieces of lens medium below the said determinedcenter and thickness factor E1, which, in combination with the saidoptical surfaces which have been formed on said pieces of lens medium,will produce the desired focal power De in the finished lens system, andsupporting said pieces of lens medium in operative relation with eachother, said blank being adapted to have the total of its centerthicknesses and separations reduced to said determined center thicknessand separation factor 21- during the forming of said rear ocular surfaceso as to introduce the focal power De desired in the lens system withsubstantially no change in the. amount of said separately controlledmagnification component.

4. The method of.making an ophthalmicle'su" System, of lens medium of anvindex of refraction such as is generally used for ophthalmic lenses,and which has a separately controlled magnification component S1 whichdistinguishes from the inherent magnification due to focal power presentin the finished lens system, with the amount of said magnificationcomponent dependent upon a factor D1 which is the combination of all ofthe surface powers of the lens system except that of the ocular surface,and a factor E1- which is the combination of the center thicknesses andthe center separations of the finished lens system, comprising choosingvalues, to make up one of said factors, which are consistent with theweight, appearance, and optical characteristics desired in the finishedlens system, calculating the values to make up the other of said factorsby means of the formula 1 sl- C7 being a function of all the surfaces,refractive indices of the glass, thicknesses and separations, except thepower of the ocular surface, according to the indices of refraction a ofthe pieces of lens medium used for said system, the desiredmagnification component S1, and the said chosen values which make up thefirst of said factors, forming optical surfaces on all of the surfacesof the lens system, except the ocular surface, to curvatures simulatingthe surface power considerations p of the calculation, on pieces of lensmedium of said indices of refraction which have center thicknesses andseparations 1- which together make up a total center thickness which isgreater than the said determined center thickness and separation factorE-r of said calculation by an amount which is such that after the saidoptical surfaces have been formed on all of the surfaces of the lenssystems except the ocular surface thereof, the total of the centerthicknesses and separations of said pieces of lens medium will still besufficiently greater than said determined center thickness andseparation factor 21 to allow the forming of a rear ocular surface onone of said pieces of lens medium without reducing the total of thecenter thicknesses and separations of said pieces of lens medium belowthe said determined center and thickness factor 2r, forming said rearocular surface on one of said pieces of lens medium to such a curvaturewhich, when combined with the said optical surfaces which have beenformed on the said pieces of lens medium will produce the desired focalpower De inv the finished lens system and supporting said pieces of lensmedium in operative relation with each other, said lens system beingadapted to have the total of its center thicknesses and separationsreduced to said determined center thickness and separation factor 2fduring the forming of said rear ocular surface so as to introduce thefocal power De desired in the lens system, with substantially no changein the amount of said separately controlled magnification component S1,said steps of said method being taken in desired order.

EDGAR D. TILLYER.

